1. Field of the Invention
The present invention relates to a novel material that can be applied to a light-emitting element. In addition, the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device each including the material.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the substance having a light-emitting property.
Such light-emitting elements are classified into a self-luminous type, and thus have advantages over liquid crystal displays, such as high pixel visibility and the eliminated need for a backlight. Accordingly, such light-emitting elements are thought to be suitable as flat panel display elements. The light-emitting elements have another great advantage that they can be manufactured to be thin and light. Further, very high speed response is one of the features of such elements.
Further, since such a light-emitting element can be formed in a film shape, plane light emission can be easily obtained. Therefore, a large-area element capable of the plane light emission can be fainted. This is a feature which is difficult to obtain from point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Accordingly, the light-emitting elements using EL have a great deal of potential for use as planar light sources which can be applied to illumination and the like.
Light-emitting elements using EL can be roughly classified in accordance with whether the light-emitting substance is an organic compound or an inorganic compound. In an organic EL element including a layer containing the light-emitting organic compound between a pair of electrodes, voltage application to the light-emitting element causes electrons and holes to be injected from a cathode and an anode, respectively, into the layer containing the light-emitting organic compound, and current flows. As a result of the injection of both electrons and holes, the light-emitting organic compound is excited, and when the light-emitting organic compound returns to a ground state from an excited state, the light-emitting organic compound emits light.
Having such a mechanism, the above-described light-emitting element is called a current-excitation light-emitting element. Note that the excited states formed by an organic compound include a singlet excited state and a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, whereas luminescence from the triplet excited state is referred to as phosphorescence.
There are many problems which depend on substances in improving element characteristics of such a light-emitting element. Therefore, improvement of an element structure, development of a substance, and the like have been carried out in order to solve the problems. For example, Patent Document 1 discloses a light-emitting element in which a compound having an anthracene skeleton is used as a light-emitting material. However, it cannot be said that the light-emitting element has sufficiently high reliability.
In addition to light emission by recombination of carriers excited with a current, there is also a method of light emission in which excitation energy is transferred from an organic compound excited with a current to another organic compound and accordingly the latter organic compound is excited to emit light. This method is effective in the case where the emission efficiency is reduced (concentration quenching) due to stacking interaction caused by a high concentration of organic molecules that are desired to produce luminescence. In organic EL elements, the method is generally applied to the element structure used in which a light-emitting material is dispersed in a light-emitting layer (a light-emitting layer is doped with a light-emitting material). Doping a host material with organic molecules that are desired to emit light suppresses the stacking interaction, whereby efficiency of a light-emitting element can be increased. In such a light-emitting element, excitation energy is transferred from a host material excited by current excitation to a dopant material, making the dopant material emit light. Note that when Substance A is dispersed in a matrix formed of Substance B, Substance B forming the matrix is called a host material while Substance A dispersed in the matrix is called a dopant material.
Among these dopant materials, types of material that emits blue light are fewer than those of material that emits light of a color having a long wavelength (e.g., red, orange, yellow, or green). Among them, favorable materials are few. It is because blue light emission needs a material with small conjugation, and thus, there is limitation on a skeleton to be selected. In addition, it is also because blue light emission needs a higher energy than light emission of a color having a long wavelength, and the high energy easily degrades a dopant material.
From the above, a material for a blue light-emitting element is desired in order to provide a highly reliable organic EL element that emits favorable blue light.